1. Field of the Invention
The field of the invention is that of pneumatically driven devices and, more particularly, air tools driven by one or more air cylinders.
2. Description of the Prior Art
Many types of pneumatically driven devices including tools are known in the prior art. Some of these tools include sanding and filing devices wherein a shoe is reciprocated by way of one or more pistons. In number of known prior art devices which have been commercialized a piston carries a rack and a rack is mounted on a shoe, the piston driving a pinion gear which meshes with the rack on the shoe for driving it. In some constructions there are two pistons, each with a rack and each driving a separate pinion meshing with a rack on the shoe that is driven. Typical of known prior art devices of this type are those shown in U.S. Pat. Nos. 3,399,494; 3,214,823; 3,563,134; and 3,732,651. Typically, these devices are subject to the deficiency that when a piston is driving a reciprocating shoe, it, of course, develops momentum during its stroke and it must come to a full stop. The momentum of the piston has to be dissipated by way of its engagement with the teeth of the pinion gear which engages the rack on the shoe. This causes excessive wear on the teeth of the pinion and the teeth on the piston rack that necessitates early maintenance of the tool. It is even possible for the driven piston to be driven out through the end of its cylinder bore.
In the prior art it has been known to provide an effective sealing of a piston by way of a sealing ring fitting in an annular groove and with ports to communicate pressure to a position behind the ring to push it out for sealing. In the prior art this construction has appeared only at the end of the cylinder which acts against the driving pressure. This type of construction has been known in the patents, including U.S. Pat. Nos. 1,492,043; 2,332,763; 2,194,102.
The herein invention overcomes the deficiencies referred to in the foregoing in the manner described in detail hereinafter.